1. Field of the Invention
The present invention relates to a method for decoding at least one signal transmitted by means of at least one antenna and received by means of at least one antenna, which method includes a symbol decoding step for producing at least one estimated symbol representative of at least one transmitted symbol carried by the received signal.
2. Description of the Background
Such signals are exchanged in telecommunication systems of the Multiple Input Multiple Output type, further referred to as MIMO systems. A main feature of MIMO systems lies in the fact that a plurality of antennas may be used both at a transmitter end and at a receiver end of a wireless link. MIMO systems have been shown to offer large transmission capacities compared to those offered by single antenna systems. In particular, MIMO capacity increases linearly with the number of transmitting or receiving antennas, whichever the smallest, for a given Signal-to-Noise Ratio (SNR) and under favourable uncorrelated channel conditions. Specific coding schemes have been designed to exploit such an increased available transmission capacity. These schemes, called space-time codes, mainly aim at transmitting signals that are redundant in space and time, which means that a same information shall be transmitted over several antennas and several times, in order to benefit from the spatial diversity offered by the multiple antennas. Several types of space-time codes, designed according to various criteria, can be found in the literature.
Due to the advantages described above, MIMO techniques are likely to be used in future wireless systems intended to provide large spectral efficiencies or, alternatively, reduce the transmitting power required for obtaining a spectral efficiency equivalent to that which is obtained in current telecommunication systems. Such MIMO techniques will very likely be combined with multi-carrier modulation techniques like OFDM (standing for Orthogonal Frequency Duplex Multiplex) and MC-CDMA (standing for MultiCarrier-Code Division Multiple Access), which are also likely to be used in future wireless systems. In specific embodiments of MIMO systems, the information to be transmitted may be encoded with respect to space and time in a manner allowing to use only one antenna at the receiver end.
In the present state of the art, several aspects of space-time encoded MIMO systems are still open issues, such as symbol detection schemes to be used on the receiver end of a signal transmitted by a transmitter using multiple antennas. Indeed, such a signal must be decoded by the receiver by means of a space-time decoder presenting a complexity which should be as low as possible. Among existing space-time coding schemes, only Space-Time Orthogonal Block Codes (STOBC) enable optimal symbol decoding with reduced complexity. However, STOBC are suboptimal, in terms of mutual information for a channel rank bigger than one, which is often the case when there is more than one receiving antenna. However, the above-described advantages of MIMO systems are in principle obtained by using more than one receiving antenna.
More spectrally efficient schemes such as Bell Labs lAyered Space Time codes (BLAST) or, more generally, Linear Dispersion Codes (LDC) involve a symbol decoding complexity, which may, in the case of maximum likelihood decoding, increase exponentially with the spectral efficiency, and execution of true maximum likelihood or a posteriori probability (APP) decodings appears unrealistic for high spectral efficiencies. This is annoying when MIMO techniques are used for addressing large spectral efficiencies.
List Sphere Decoding (LSD) provides nearly optimal APP decoding, but involves so much computing power that its practical implementation in a terminal which is to be power-fed by a battery still appears unrealistic in many cases.